Propeller blade



1952 e. 'r. LAMPTON ETAL 2,620,885

PROPELLER BLADE Filed March 28. 1947 2 Sims-MT 1 INVENTOR Erie Martin Gian 7T Lampiou mim ATTORNEY Patented Dec. 9, 195 2 PROPELLER BLADE Glen T. Lampton and Erle Martin, West Hartford, Conn., assignors to United Aircraft Corporation, East Hartford, Conn, a corporation of Delaware Application March 28, 1947, Serial N o. 737 ,846

Claims. 1

This invention relates to propellers, and particularly to propellers having substantially square tip portions.

Propeller tests and the data derived therefrom have in the past dictated that to provide the most efficient propeller, propellers, and particularly metal propellers, should have a plan form tapering from approximately the mid-length toward the tip so as to produce a relatively na r row tip portion and should have a rounded tip end. In consequence substantially all of the metal propellers which have been manufactured in recent years have had the tapered and rounded tip plan form.

We have discovered however, that aeronautical propeller blades formed with wide tip portions and substantially square across the tip end, that is, having the tip terminate in a substantially straight line at substantially right angles to the longitudinal axis of the blade, will provide a propeller substantially as eflicient and in some cases, more efficient than the design previously considered best. This square tipped propeller blade has been found to be easier to manufacture than the rounded tip propeller, and in some forms of metal propellers, has been found to effect substantial economies in manufacture over the rounded tip propeller.

An object of this invention is an improved and more efficient areonautical metal propeller.

A further object of this invention is a propeller which will delay the formation of compressibility waves at the propeller tip at speeds near the speed of sound.

A further object is a method of manufacturing propellers which will result in substantial economies in manufacture and will produce an efiicient propeller.

These and other objects and advantages of the invention will be apparent from the specification and claims, and from the accompanying drawings which illustrate what is now considered to be a preferred embodiment of the invention.

In these drawings,

Fig. 1 is a plan view of a propeller constructed in accordance with this invention. In order to show the plan form of the blade, the normal twist of the blade has been removed so that the view is in effect, a developed view.

Fig. 2 is a side elevation of the blade of Fig. 1.

Fig. 3 is an enlarged cross section on line 3-3 of Fig. 1 showing the internal construction of the tip end of the hollow steel blade of Fig .1.

Fig. 4 is a cross section on the-line 4-4 of Fig.

2 1 and Fig. 5 is a cross section on line 55 of Fig. 1. The angles from the horizontal at which the sections of Figs. 4 and 5 are arranged indicate a normal pitch angle of the blade at those sections.

Fig. 6 shows a hollow steel blade, with the twist removed, of slightly different plan form than that shown in Fig. 1, and Fig. '7 is an enlarged cross section on the line 1! of Fig. 6 showing a detail of the blade tip construction.

Fig. 8 shows the plan form, with the twist taken out, of another form of blade such as a solid aluminum alloy blade.

Fig. 9 is a chart showing the curve of maximum thickness of the propeller of Fig. 8.

Fig. 10 is a section on line lll-| 0 of Fig. 8. The angle of Fig. 10 from the horizontal is an indication of the pitch angle of the blade.

Fig. 11 is a plan view, with the twist taken out, of another form of steel blade. This blade is schematically shown as arranged in a forming die.

Fig. 12 is a side elevation of the propeller and die of Fig. 11.

Fig. 13 is a section on the line l3l3 of Fig. 11. The angle from the horizontal at which the blade section is shown in Fig. 13 is an indication of the twist in the blade from the shank end of the blade to the particular section, and is an indication of the angle at which the blade is arranged in the die.

As herein shown, Fig. 1 illustrates diagrammatically, a hollow steel blade which may be made in accordance with the process set forth in Lampton application Serial No. 484,254, filed April 23, 1943, which issued as Patent No. 2,511,858, and may be a propeller of the kind described in Martin application Serial No. 484,229, filed April 23, 1943, which issued as Patent No. 2,511,862. Reference may bemade to these two applications for a more detailed explanation of the construction and process of manufacture of these steel blades. The blade comprises a core 20 which may be a steel tube flattened and closed at the tip end 22 and having blade retaining means 24 at the other end. The blade retaining means may be of a type described in Anderson Patent No. 2,315,574, for Propeller Blade Mounting issued April 6, 1943, in which a series of ball races are ground into the core member 26. The core end 24 is adapted to be received in the socket of a propeller hub (not shown) and held in position therein by a plurality of ball bearings in the ball races in the blade end and in cooperating ball races in the hub socket. Such a construction permits blade pitch variation and control by well known pitch control means. The hub carrying the hub socket is adapted to be mounted on an airplane engine in a well known manner for the purposes of driving the propeller and furnishing propulsion to the aircraft. A shell member 26 preferably formed of sheet steel is secured to the core member preferably by means of brazing.

As explained in the above-identified Larnpton application, the shell is preferably preformed by blowing it at an elevated temperature in a heated die and it is preferably brazed to the preformed core member while the shell and core are held in assembled relation in a heated die. In the blowing process, the steel shell is held in position in the die by any desired indexing means such as a tab (not shown) formed integral with the shell and registering with a cooperating part of the die while gas under pressure forces the heated shell against the die impression.

As shown in Fig. 3 the tip of the core and the tip of the shell may be provided with a fillet, usually of copper, such as fillets 28 and 30 in the shell and core respectively. When the fillets are used the outside seam of the shell such as that shown at 32 and 34 in Fig. 7 may be removed to provide a smooth rounded exterior. In Fig. '7 the copper fillet is not used, but the seam weld in the portions 32 and 34 is relied upon to hold the opposite sides of the blade tip together. Drain holes 36 and 38 are provided in the blade shell and core respectively.

It will be noted that the blade in Fig. 1 is substantially as wide at the tip portion as it is at the mid-section. It will be noted that the blade thickness as shown in Fig. 2 remains substantially the same from the tip portion back towards the shank portion for a considerable distance, that is, about a fourth of the blade length. This thickness is made as small as is consistent with strength. By making the blade width, or chord length, substantially the same as it is at the midsection, it is possible to give the blade a thinness ratio at the tip, that is, a ratio determined by dividing the chord of any section by the thickness of that section, that is considerably greater than would be possible with a blade of the same thickness but with a smaller chord, as in the case of a tapered blade.

It is well known that in general the lift of a wing varies with the mean camber of the section and with the angle of attack. Camber of a wing section may be defined as the maximum distance between, and normal to, a straight line connecting the leading and trailing edges of a wing section (1. e. chord) and a median line located midway between the camber and face surfaces of the wing. The drag of the section however, also increases with both the increase in mean camber and increase of angle of attack. With increases in speed, limitations are imposed on both camber and angle of attack due to the increase of drag with speed and also due to the tendency of the wing to stall or the airflow to break away from the wing and thus reduce its lift at the increased speed. The object of any wing surface, such as a propeller blade, is to obtain maximum lift with minimum drag, under the conditions of operation. We have found that at speeds approaching the speed of sound it is desirable, in order to obtain maximum lift with minimum drag, to give the propeller blade a shape such that at the tip the camber approaches zero (i. e. approaches a symmetrical section). The lift is then obtained largely by virtue of the angle 4 of attack, which at high speeds is the more efficient way and delays the formation of shock waves.

By making the blades wide at the tip we are able to obtain a large area of efficient blade surface having a large thinness ratio and small camber.

In propeller design, power absorption, speed, and diameter are important. It is desirable to obtain the greatest possible power absorption, at the highest speed and mallest diameter. A wide tipped blade having a small camber and large thinness ratio will fulfill these requirements.

By giving the blade a wide tip section with small, or no camber, it is possible to carry this small camber feature well into the blade sections toward the shank. It is thus possible to give substantially the entire blade a smaller camber, where the wide blade tip has a small camber, than is possible in the narrow tipped relatively large camber blades. This makes for a more efficient blade.

This thinness ratio and camber of the airfoil becomes of particular importance in high speed airplanes where the blade tip approaches the speed of sound. The greater thinness ratio and lower camber both contribute to delay the formation of shock waves and a greater propeller speed is possible before shock waves are formed and the propeller is more efficient after shock waves are formed. By widening the blade tip to give increased power absorption characteristics thus permitting a reduction in camber on the intermediate blade sections it is possible to delay the formation of shock waves over the cambered surface of the intermediate sections as well as at the blade tip. Greater propeller efficiency at high speed is therefore possible. By making the propeller blades as shown in Fig. 1 with the tip portion terminating in a substantially straight line substantially normal to the longitudinal axis of the blade as indicated by the line 40, it is possible to give the blade tip a well-defined predetermined airfoil shape which is determined by the forming dies in which the shell is formed. This airfoil shape becomes of particular importance in the high speed airplanes as slight variation from the optimum form will materially and adversely effect the blade efficiency.

While Figs. 1 to 7 show this invention incorporated in a hollow steel blade, Figs. 8 to 10 show the invention incorporated in an aeronautical propeller blade of the solid type, such as a solid aluminum alloy blade. In the solid blade the shank end is flanged in the usual manner to support the bearing elements 42 by means of which the propeller is retained in a well known manner in a propeller hub. The blade then flares from a round shank section into an airfoil section 44 which may be widest at approximately the midlength or may be substantially the same width at the tip as it is at the mid-length or may even be wider at the tip than it is at the mid-section. In Fig. 9 the distance between line 66 and base line 68, normal to line 68, indicates the maximum thickness of the blade section at the corresponding portion of the blade of Fig. 8. It is important that the thinness ratio be maintained as large as practicable and that the tip be formed as near an optimum airfoil section as possible.

In both the steel and aluminum blades the square tip portion provides a blade having a comparatively large effective area at the tip or outer half of the propeller blade where the pitch angle is favorable and can efficiently produce an effective thrust while the portion of the blade near the shank, which of necessity, must be arranged at a large pitch angle and therefore although creating considerable drag, does not result in a substantial amount of thrust, is comparatively small. This type of blade construction results in a blade of increased over-all efficiency particularly on high speed airplanes where the propeller blades are necessarily set at relatively high pitch angles.

Figs 11, 12 and 13 showing a propeller blade of substantially rectangular plan form schematically show the dies 66 in which the blade may be formed. A method by which these blades may be formed is described in the above mentioned Lampton application Serial No. 484,254, to which reference is made for further details in forming the steel blade and particularly the shell portions thereof. The blades may be indexed in any desired manner such as by a tab (not shown) extending from one edge of the blade into a registering portion of the die. The blade shown in full lines in Fig. 11 occupies the entire blade depression in the die 65 and is the largest blade which may be made in that die.

Hollow steel blades of the type herein described cannot be ground to shape, as is usual with solid propellers, because the airfoil forming shell is of comparatively thin sheet material. Shorter blades can not be made from longer ones simply by cutting oif a portion of the end of the longer hollow blade because the entire end of both core and the shell would then be open.

If shorter blades, which are necessary to pro vide a propeller of a small diameter, are desired, shells may be formed of any selected shorter length as indicated by the dot and dash lines 45, 48, 50, 52, 54, 5B, 58, 65 and 62. correspondingly shorter cores would of course be used. The shorter shell is seam welded in the usual manner and placed in the die with the shank end in the same place as the longer shell would occupy although the entire depression is longer than the shell. The indexing means will hold the shell against longitudinal movement in the die and thus properly position the shell in the die so that regardless of where the blade shell terminates, it will take the form of the die at the terminating point and thus accurately form the blade tip section with the optimum airfoil form. The shorter blades of a series formed in a particular die will have a somewhat smaller thinness ratio than the longer blades formed in that die as the blades taper a little in thickness, increasing toward the shank while the chord length may remain substantially the same. This slight variation in thinness ratio is however, not material and the smallest ratio is still greater than the thinnesss ratio possible with the tapering and rounded tip blades. In practice, it has been found that a thinness ratio of over 20 to one is quite possible, ratios of between 20' and 30 to 1 having been produced at the tip portion of the square tipped blades. It is thus possible to form a series of different length propeller blades in a single die which results in considerable economy in the manufacture of the blades, as the dies are quite expensive. On the other hand, if blades were to be formed with a rounded tip as has been the custom, a separate die for each blade length would be required and it would render the mak ing of the hollow steel blades an expensive process. It is obvious that if a round tip blade were to be formed in a die designed for a longer blade, the tip portion of the shorter blade would not be a true airfoil structure at the leading and trailing edges and would therefore be quite inefiicient.

In the forming of solid aluminum alloy blades, the rough forgings are ground to shape by hand, using a template to determine the correct shape, or by machine. The shorter blades are usually made by cutting off the tip portion of a, longer blade. Where the tip portion has to be rounded, it is necessary to form the tip portion by hand so that theycontour is uncertain. Where, however, the blades are formed with a square tip in accordance with the present invention, the shape determined by the template or by the grinding machine need not be altered but it is only necessary to round off the corners where the blade is cut off.

It will be evident that as a result of this invention, a propeller blade has been produced which is more efficient than the current blades and which even in the steel blades, can be manufactured more accurately and economically than the rounded tip blades.

While the preferred embodiments of the invention have been shown and have been described and illustrated in the accompanying drawings, it will be understood that these embodiments of the invention are shown for purposes of illustration only and that various changes in the construction and method of manufacture may be made without diverting from the spread and the scope of the invention as defined by the appended claims.

We claim:

1. A propeller blade of airfoil section gradually decreasing in thickness toward the blade tip and terminating in an airfoil shaped portion adjacent said tip substantially as wide as the blade midway of the blade length and having a thinness ratio of over 20 to 1, said portion terminating at the blade tip in a substantially straight edge substantially normal to the longitudinal axis of the blade.

2. A propeller blade as defined in claim 1 in which the tip portion has substantially zero camber and tapers in thickness toward the leading and trailing edges from a point therebetween.

3. A hollow aeronautical propeller blade gradually decreasing in thickness toward the blade tip and comprising an airfoil forming tubular shell having a tip portion whose chordwise dimension is substantially as great as the chordwise dimension midway of the blade length, said blade tip portion having a thinness ratio of over 20 to 1 and terminating in a substantially straight edge substantially normal to the blade axis.

A propeller blade as defined in claim 3 in which the ti portion has substantially zero camber and tapers in thickness toward the leading and trailing edges from a point therebetween.

5. A propeller blade tapering in thickness toward the leading and trailing edges from a point therebetween and having a camber surface and a face surface, said blade having a chordwise dimension at the tip substantially as great as the chordwise dimension midway of the blade length and having a thinness ratio greater than 20 to 1 and substantially zero camber at the blade tip and gradually increasing camber toward the blade shank whereby the air speed over the tip portion and over said section will be substantially equal.

GLEN T. LAMPTON. ERLE MARTIN.

(References on following page) 7 8 REFERENCES CITED .Number Name Date The following references are of record in 'the 2394345 1946 file of t i t t: Han'dler Feb. 5, 7 2,408,788 Ludington et a1. 'Oct. 8, 1946 UNITEDSTATES'PATENTS 5 2,465,007 Bragdon Mar. 22, 1949 Number Name Date .Re,l21 ,382 Stuart Mar. 5, 1940 FOREIGN PATENTS 913,951 Gowing Mar, 2, 1909 Number 901mm Date 1',777;013 Ness Sept, 30, 1930 28,253 Great'Brltain 1909 2950142 'w t Aug 4, 11936 10 519,759 France June 1921 2,090,888 Gill Aug. 24, 1937 OTHER REFERENCES 2,173,397 Littlejohn Sept. 19, 1939 2262163 fBrauchler NOV 11 1941 "fF1ig'ht magazine forlNov. 4,1937, pp. 450-452. 

